AC1740 Model R6350 & Xfinity Modem

You have a router at the ISP modem level already. The 6350 series is years old and of older generation WiFi that doesn't have the capabilities to support wifi speeds much past 300Mpbs. So if you want to see anything past 300Mpbs, you'll need to replace the R series router with something newer generation:

https://www.netgear.com/home/wifi/routers

Also, if your ISP Modem already has a built in router and wifi. This would be a double NAT (two router) condition which isn't recommended. https://kb.netgear.com/30186/What-is-Double-NAT

https://kb.netgear.com/30187/How-to-fix-issues-with-Double-NAT

Couple of options,
1. Configure the modem for transparent bridge or modem only mode. Then use the Orbi router in router mode. You'll need to contact the ISP for help and information in regards to the modem being bridged correctly.
2. If you can't bridge the modem, disable ALL wifi radios on the modem, configure the modems DMZ/ExposedHost or IP Pass-Through for the IP address the Orbi router gets from the modem. Then you can use the Orbi router in Router mode.
3. Or disable all wifi radios on the modem and connect the Orbi router to the modem, configure AP mode on the Orbi router.
Static IP Address Configuration for AP Mode | NETGEAR Communities

https://kb.netgear.com/31218/How-do-I-configure-my-Orbi-router-to-act-as-an-access-point and https://www.youtube.com/watch?v=H7LOcJ8GdDo&app=desktop

Try option #2 first...
https://kb.netgear.com/000061277/Which-features-are-disabled-on-my-Orbi-router-when-it-is-set-to-AP-Mode

Good Luck